Looking at antirelayd, you probably can do, but they could easily be messed up by the script that uses that file - /usr/sbin/antirelayd

And indeed, you don't need to restart antirelayd if you change the file as the script reloads it on each daemon run by the looks of it. The file /etc/alwaysrelay isn't actually used by exim itself, it's used to populate /etc/relayhosts by antirelayd.